The above mentioned copending application relates to a high performance secondary electrochemical battery or cell particularly having a negative electrode of a lithium-aluminum alloy and an additive of graphitized carbon or Raney iron, the composite electrode exhibiting increased capacity and lithium-alloy utilization when compared to a lithium-alloy electrode without the additive.
In prior developed high temperature secondary electrochemical cells, the positive electrode generally has been formed with calcogens such as sulfur, oxygen, selenium or tellurium, as well as their transition metal chalcogenides: while the positive electrode materials have included the sulfides of iron, cobalt, nickel and copper.
In high temperature cells, current flow between electrodes often is transmitted by molten electrolytic salt. Particularly useful salts include compositions of the alkali metal halides and/or the alkaline earth metal halides ordinarily incorporating a salt of the negative electrode reactant metal, that is lithium. In cells operating at moderate temperatures, aqueous and organic base electrolytes are permissible and these also can include cations of the negative electrode metal.
Alkali metals such as lithium, sodium, potassium or alkaline earth metals including calcium, magnesium, etc. and alloys of these materials are contemplated as negative electrode reactants. Alloys of these materials such as lithium-aluminum, lithium-silicon, lithium-magnesium, calcium-magnesium, calcium-aluminum, calcium-silicon and magnesium-aluminum have been investigated to maintain the negative electrode in solid form and thereby improve retention of the active material at the high operating temperatures of the cell.
One of the disadvantages of previous lithium-aluminum electrodes, has been the reduction in cell capacity during prolonged operation. The reduced capacity has been accepted in order to obtain the enhanced electrode and cell stability afforded by solid lithium alloys. In the lithium-aluminum negative electrode, postoperative examination of long-lived cells have revealed high lithium concentrations at the negative electrode face and agglomeration of lithium aluminum (Li-Al) particles in the porous electrode.
The basic lithium-aluminum/iron-sulfide battery has the negative electrode formed of a lithium-aluminum alloy while the positive electrode is formed of iron sulfide (FeS or Fe.sub.2 S). An electrolyte comprised of blended lithium chloride and potassium chloride (LiCl, KCl) encompasses both electrodes and the space intervening. During charging of the battery, a chemical reaction provides that lithium plus an electron migrate toward the negative electrode to form lithium aluminum, while iron and lithium sulfide less an electron form iron sulfate in the positive electrode. The discharge cycle conversely provides that lithium ions migrate towards the positive electrode and react with the iron-sulfide to form lithium sulfide (Li.sub.2 S) and iron.
The lithium aluminum/iron sulfide electrochemical battery is being proposed for use with electric vehicles where a specific high power level is needed as well as where repeated charge and deep discharge cycles will take place (up to perhaps 75-90% depth of discharge). With this deep discharge, the internal resistance of each battery cell increases dramatically to reduce the output power up to 40-60% when compared to the fully charged condition.
In analyzing this phenomenon, the internal resistance of the positive electrode at 75-90% depth of discharge has been noted to increase to three times that at the fully charged condition; while the internal resistance of the negative electrode at the same 75%-90% depth of discharge has been noted to increase by 20-30% compared to the fully charged condition. Furthermore, the positive electrode discharges to a greater percentage of the full capacity potential even after many repeated charge/discharge cycles; whereas the maximum potential capacity of the negative electrode drops off slightly for each charge/discharge cycle so that after the same number of such cycles, the effective fully charged potential capacity of the negative electrode might be only 75% of that when brand new. This repetitive deep discharge cycling of the negative electrodes thus incrementally reduces the overall capacity of the cell, or of a battery in a multiple cell hook-up.
For these reasons and others, the design strategy in a lithium-aluminum/iron-sulfide battery has been to make the negative electrode of significantly greater capacity than the capacity of the corresponding positive electrode, so that during the deep discharge conditions of the positive electrode, the negative electrode yet has ample reserve capacity remaining. This might be typified by a positive electrode having a design capacity of perhaps 250-275 amp. hrs. and a negative electrode having a design capacity of 325-360 amp. hrs; whereby the negative electrode has 10-25% overcapacity compared to the positive electrode and the battery is classified as a positive electrode limited design.
The above referred to copending application Ser. No. 287,857 disclosed that the addition of a small amount of graphited carbon or Raney iron (the intermetallic Al.sub.5 Fe.sub.2), or mixtures thereof, to the lithium-aluminum electrode unexpectedly increased the cell capacity and stability over multiple deep discharges and significantly increased the lithium aluminum uitilization.
The graphitized carbon, as referred to herein, is particulate carbon which is heated in a protective atmosphere to a temperature in the range of about 1000-2000.degree. C. in order to initiate the change to graphite structure. Higher heating temperatures are possible and not excluded but are deemed unnecessary. Graphitized carbon as used herein does not require that the carbon be graphitic. The carbon added to the negative electrode is to be present in the range of about 1-10 volume % of the electrode, preferably in the range of about 3-7 volume % of the electrode.
When Raney iron (Al.sub.5 Fe.sub.2) is used in place of the graphitized carbon, it is preferred that the Raney iron be present in the range of about 3-10 volume % of the electrode.
Specifically, the preferred negative electrode has a lithium content in the range of about 5-50 at. % and an aluminum content in the range of from about 95-50 at. %. The negative electrode is formed of particulate mixture of lithium-aluminum alloy and the electrolyte, and a material selected from graphited carbon, an aluminum-iron alloy and mixtures thereof, the lithium-aluminum alloy being present in the range of about 45-80 volume % of the electrode, the electrolyte being present in an amount not less than about 10 volume % of the electrode, the graphitized carbon being present in the range of about 1-10 volume % of the electrode, and aluminum-iron alloy being present in the range of about 3-10 volume % of the electrode. The positive electrode comprised the active material of a chalcogen or a transition metal chalcogen.
In the fully charged state, the potential between the negative and the positive electrodes is approximately 1.30 volts. In the over-discharged state of a positive electrode limited cell, the potential between the lithium deposited in the positive electrode and the aluminum remaining in the negative electrode is about a negative 1.0 volts. This phenomena of reversing the polarity of the electric output is known as voltage reversal. The impact is quite evident in a multiple cell battery where if one cell is totally discharged and is driven beyond the 100% discharge range by the adjacent cells, the output voltage of the battery is then reduced by an amount greater than the voltage normally contributed by the defective cell when it otherwise was in good operating condition.